An existing field device adapted to determine a process parameter value related to a tank, such as Rosemount 5400 Radar Level Transmitter or Rosemount 5300 GWR Level Transmitter, comprises a terminal block with an interface configured to connect to two wires of a process control loop for power supply to and signal output from the field device.
There may however be a need to connect an external device to that type of field device. The external device may for example be a remote display. When connecting to an external device, complying with Ex (explosion protection) requirements and how to supply power to the external device may have to be considered.
U.S. Pat. No. 7,262,693 (Karschnia et al.) discloses a process controller monitoring system which includes a field device which connects to a control system through a process control loop through a junction box. Another field device couples to the process control loop and includes wireless communication circuitry. The wireless communication circuitry is configured to send an RF signal and to be completely powered by power received from the process control loop. The other field device includes a power regulator, a shunt or bypass, and a super capacitor. During operation, the super capacitor is slowly charged using a power regulator by using excess voltage tapped from the process control loop. The use of a capacitor allows quick charging and the storage of sufficiently large energy potentials. However, when used in a hazardous environment, large energy storage may not be accepted in order to meet intrinsic safety standards. In such a case, U.S. Pat. No. 7,262,693 teaches that the other field device is moved away from the hazardous environment, to the junction box where intrinsic safety is not required. Furthermore, the other field device reserves power even if the wireless communication circuitry is not used, which may impair the lift-off/update rate at all times.